How to display and use operator properties in a custom UI panel?
You can set new property like this:

bpy.types.Scene.my_use_x=bpy.props.BoolProperty()
row.prop(context.scene, 'my_use_x')


And set property of operator like this:

operator = row.operator("mesh.vertices_smooth")
operator.use_x = context.scene.my_use_x
operator.use_y = False
operator.factor = 2


I do not use blender 2.8 and I did not test this. But it should work.